Smartphone Elements: Metalloids and Rare Earths

Note to readers: The post formerly titled ‘Smartphone Chemistry: An Embarrassment of Riches‘ was poorly titled and has been disappeared. This is an updated version and one titled more appropriately.

The modern smartphone is made from many different chemical elements, some much more scarce than others. The elements found in a smartphone are distributed around the world, with some countries being more favored by geology than others. With perhaps the exception of silica and silicon, the elements below are found in localized ore bodies that are enriched in particular elements in the form of minerals. ‘Enriched’ is a relative term meaning anywhere between a few 10s of percent to 100 parts per million or less. The word ‘enriched’ also implies another attribute wherein the extraction of the desired element is economically feasible. With the exception of carbon listed below, most of the elements are metals or metalloids. Metalloids are elements that are not entirely metallic but not entirely non-metallic either. Many are found in the p-block of the periodic table.

This intermediate chemical nature of the elements we call metalloids may seem a bit dodgy and imprecise, but the term metalloid, while not precisely defined, is thriving out in the big, big world. The metalloids are found in the p-block of the periodic table. Below is a partial chart of p-block elements. Elements polonium and astatine are too rare and too radioactive to be of any practical relevance. It seems as though there is some disagreement as to which elements should be in the set of metalloids.

Here is the deal with metalloids. While they may not be used for their physical properties as other metals like in bridge or shipbuilding, their electronic properties provide valuable utility to civilization. By ‘electronic’ I specifically refer to the valence level electrons around the atoms and how they interact.

‘Valence-level’ refers to the outermost layer of atomic or molecular orbitals that are partially filled with electrons. It is at the valence-level where interactions with other atoms and molecules take place. Both the valence-level electrons and empty orbitals are where reaction chemistry occurs.

Source: http://www.compoundchem.com. Copyright/ Andy Brunning 2023 | Creative Commons Attribution-Noncommercial-NoDerivatives License.

The black element boxes above represent the Rare Earth Elements (REE) and includes the whole 15 element lanthanide series plus scandium (Sc) and yttrium (Y). Note that Sc and Y are in the same column as lanthanum which is the beginning of the lanthanide series. Generally, elements in the same column share certain chemical properties like in this case the +3 oxidation state, so this is why Sc and Y are considered by some to be in the REE group. The truth is that REE is a woefully antiquated term that just won’t disappear.

“The “rare” in the name “rare earths” has more to do with the difficulty of separating of the individual elements than the scarcity of any of them.” [Wikipedia]

“..,. these elements are neither rare in abundance nor “earths” (an obsolete term for water-insoluble strongly basic oxides of electropositive metals incapable of being smelted into metal using late 18th century technology.” [Wikipedia]

All REEs share the +3 oxidation state, but some of them can have other oxidation states as well. Samarium, europium, thulium and ytterbium can be in the +2 and +3 oxidation states. Cerium, praseodymium, neodymium, terbium and dysprosium all have the +3 and +4 oxidation states. The dissimilarities do not end there. Of the lanthanides, the bookend elements lanthanum and lutetium are often not counted as REEs. The reason is that lanthanum has zero f-block electrons and lutetium has a stable, full f-block of 14 electrons, so neither participates in much f-block chemistry. Lanthanum, [Xe] 5d¹ 6s², and lutetium, [Xe] 4f¹⁴ 5d¹ 6s², may be better considered as d-block transition metals.

Snorkeling in Deeper Waters: The notation for the electronic configuration of the elements has a long and an abbreviated form. For heavier elements like lanthanum or lutetium, it is convenient use the abbreviated form showing the nearest noble gas with its orbitals filled- in this case it is xenon, abbreviated [Xe]. Now the orbital levels are filled according to a few rules that look like this for lutetium- [Xe] 4f¹⁴ 5d¹ 6s² . This shows that the f-orbitals are filled with their 14 f-electrons and that the remaining three electrons, 5d¹ 6s², are left in partially filled s- and d-orbitals. Filled energy levels (like f¹⁴) are very stable and are often quite inert. The three remaining 5d¹ 6s² electrons are available for chemistry, namely to be removed to form Lutetium (3+).

The lowest energy arrangement in which electrons naturally organize themselves under ‘ordinary’ conditions around an atom, molecule or ion is called the ‘ground state’. In the ground state all electrons occupy the lowest energy and oddly shaped regions of space called orbitals, with a maximum of two electrons per orbital. Orbitals are places, not things. There is plenty of information on this quantum chemistry business on the interwebs.

A walk on the wild side

In the image of atomic orbitals above, each orbital can ‘contain’ one or two electrons. Rather than say ‘contain’, let’s say that the orbitals describe the region of space where the one or two electrons have some probability of being found, depending on their energy. The greater the chance of finding an electron in a particular space, the greater it’s probability density. Note that the orbitals have fuzzy edges. This is because the probability density doesn’t drop abruptly to zero at the edges but rather tapers off. The Uncertainty Principle tells us that it isn’t possible to know both the momentum and the position of a particle simultaneously to high level of accuracy. It turns out that quantum mechanics can’t tell us where an orbital electron is from moment to moment. What it can do is to provide a coherent set of rules for the manner in which electrons are ‘stacked’ in the orbitals as the orbital energy changes.

Alright, we’re back

Scandium and yttrium are d-block transition metals but are sometimes lumped in with REEs because they share the Group IIIB column with lanthanum. The elements cerium through ytterbium do participate in the chemistry of f-block electrons and when REEs are spoken of, there is a good chance that the elements Ce thru Yb are the topic. Is the terminology really as higgledy-piggledy as it appears? Ah, yep.

All of the materials found in electronic devices are there as a result of performance optimization by the manufacturer’s R&D. Many elements are quite expensive, such as indium. The performance uptick from expensive elements must translate into increasing EBITDA. C-suite careers live and die by quarterly and annual EBITDA. Increased performance can be in many forms like lower power consumption, longer battery life or increased electronic performance in a chip. Chips require electrical conductors, semiconductors and non-conductors. This is the realm of material science which overlaps with chemistry.

Some of the material science challenges facing smartphone makers might seem a bit arcane. For example, when putting down a layer of material on a chip, will the substrate be wetted by the new layer so that the surfaces contact as desired? An engineering solution may require that a compatibility layer be put down first. Or do the materials have the desired dielectric constants? If you want capacitance in the device, a dielectric layer that is easy and cheap will be required. If you are doing vapor deposition, then the low dielectric material must come from a vapor phase at high temperatures. can it withstand the temperature? Do your semiconductor devices have the desired band gap? What elements affect this? What kind of chemical purity is needed for your CVD or ALD process? Four, five, six or seven nines purity (99.99 % to 99.99999 %)? The more nines of purity specified the more expensive the material and the fewer suppliers there may be.

Companies search all over the periodic table for substances that boost performance and keep Moore’s Law going. All of this must be done in a field full of patent land mines that you don’t want to step on. Invention can lead to big trouble for the unwary.